My present invention relates generally to the removal of dissolved gases and vapors from liquids to enhance and improve fluid properties. More particularly, the invention relates to a novel means and method of degassing a liquid to provide a very high purity working fluid of extremely low residual gas content.
The presence of trace amounts of gas in many liquids significantly affects fluid properties. The presence of a noncondensable gas promotes the onset of nucleate boiling in fluids, and it is strongly suspected that dissolved gases also contribute significantly in cavitation phenomena. In addition, such gases can interfere with the measurement of several fluid or system properties, including vapor pressure, virial coefficients, gas/liquid solubility, and gas/liquid diffusivity. Several methods for degassing fluids are presented in the open literature, but each suffers from difficulties in practical applications.
Of the methods for degassing a fluid, possibly the most familiar consists of partially filling a container with the fluid, freezing it, pumping away the gas above it, thawing it, refreezing, and so on for as many cycles as necessary to stabilize the value of a reference parameter such as vapor pressure.
A variation of the above-described technique with low-vapor-pressure fluids is to maintain a vacuum above the fluid, until some reference parameter stabilizes. See, for example, "Review and Modification of Experimental Technique to Determine Gas Diffusivity in Liquids" by D. Wotton et al., Proceedings of the Fifth Symposium on Thermophysical Properties, Newton, Massachusetts, September 1970.
Another fluid degassing technique utilizes a continuously refluxing distillation column. A closed vertical tube is partially filled with fluid. By heating the lower end while cooling the upper end, a two-phase reflux cycle is established. Noncondensable gas is swept by vapor flow to the cold end of the column where it forms a gas plug that can be detected by thermocouples along the axis of the condenser section. This plug is then removed through a pumping port at the extreme condenser end. Refluxing and pumping continue until no gas plug can be detected by the thermocouples.
Another method discussed in the literature is vacuum sublimation. See, for example, "An Apparatus for Degassing Liquids by Vacuum Sublimation" by T. N. Bell et al., J. Phys. Chem., 72, 1968. Fluid is frozen, then connected to a chamber which has a cold finger kept below the freezing point of the fluid. Sublimation mass transfer from the frozen pool onto the finger occurs while the chamber is being pumped. Bell et al. maintain that essentially all gas is removed during this sublimation/condensation process. A 40-cc sample takes from 1 to 2 hours to process.
These four techniques vary in sophistication, each with merits and faults. The freeze-thaw technique ultimately reaches a low gas content, but the process is inherently tedious and requires temperatures below the freezing point of the fluid, necessitating in some cases, cryogenic temperatures and associated costs.
Vacuum degassing also gives a low gas content, but suffers from large fluid fraction losses and is useful with only a very limited number of fluids with sufficiently low vapor pressure. It is attractive because it is a room temperature operation requiring only a vacuum pump.
A continuously refluxing column has good potential for scaling to large batches, but is the most ineffective method. Even when there is no detectable gas leg in the column, there can still be enough gas to affect many systems. Drawing off the gas leg can also result in significant fluid loss in the form of vapor. To achieve good separation and degassing in a short time, it is often desirable to apply high heat to the fluid pool. It is possible that attendant high surface temperatures can decompose or modify the fluid composition, possibly even producing more impurity gas.
Vacuum sublimation produces the lowest gas content of the four methods but requires, in some instances, cryogenic temperatures. The process is extremely slow for large batches because of the low mass transfer rates characteristic of sublimation. It is the least scaleable of the four processes.
A fault common to all four methods is the lack of a straightforward quantitative measure of impurity gas content during processing. At high gas levels, a pressure measurement is adequate to establish vapor composition and fluid composition if the impurity gas/liquid solubility is known, but without sophisticated instrumentation, low gas levels are difficult to detect for systems not at laboratory ambient.